The Rosetta spacecraft has made the first detailed observations of the dusty coating that formed on comet 67P on its journey through the distant reaches of the solar system.

Tiny grains collected by the European probe’s Cosima instrument found them to be fragile, rich in sodium, and made up of smaller particles that are probably the main source of the dust that astronomers can see between planets.

The European probe is circling the speeding comet as it hurtles towards the sun, giving scientists a unique chance to watch the body in close-up as its warms and starts to shed plumes of gas and dust. The comet will swing past the sun on its closest approach in August.

When the comet is near to the sun, the ice on its surface heats up and turns straight into gas, dislodging huge amounts of dust in the process. But farther out, in the colder reaches of space, the ice vaporises more slowly and leaves the dust in place. Over time, the comet surface becomes a jacket of pure dust.

Over the past four years, when comet 67P was far from the sun, a surface layer up to 12cm thick lost its ice, leaving only dust behind, said Rita Schulz, a space scientist at the European Space Agency in the Netherlands. But in the past three weeks, that coating was probably shed, revealing fresh comet material underneath.

“For the first time with Rosetta we can see a comet when it just becomes active. We have not been able to see this dusty mantle before,” said Schulz, whose study appears in the journal, Nature.

“The particles we analysed were rather fluffy and shattered when we collected them. They are made up of smaller particles, and they closely resemble interplanetary dust particles,” Schulz said. The findings suggest that interplanetary dust might originate solely from comets, and not other proposed sources, such as asteroids that break up in cosmic collisions.

Dust from the comet is collected by Rosetta when it passes through a hole in the Cosima instrument (short for cometary secondary ion mass analyser), and sticks to a target inside. The instrument photographs the particles on the target plates and then ionises them to see what elements they contain.

The Rosetta probe will study comet 67P as it swings past the sun in August and heads back out towards Jupiter.

In November, Rosetta dropped its small Philae lander on to the surface of the comet. The lander fell silent after its batteries ran out, but as the comet gets closer to the sun, Philae may receive enough sunlight to recharge its batteries and resume work on the comet’s surface.